The living substance of a uni-cellular organism, similarly the congery of cells forming the body of man, has evolved the power of sensing and of moving towards, or away from life-giving or destroying sources of energy. Special sense-organs, receptive of one or other form of energy have been evolved through the aeons of the struggle for existence, together with nervous and muscular systems, to enable him to preserve his life in the midst of the shocks and thrills of his environment. There are also evolved inner senses, and a sympathetic nervous system which knits all parts of the body in harmonious action; the community of action also being brought about by the circulating fluids of the body, the blood and the lymph, to which each living cell gives and from which each cell takes. For communion with the environment, eyes for visual, and sense organs in the skin for thermal radiant energy have been perfected, ears for sound waves travelling through air, taste organs for substances in solution, smell organs for particles of substances floating in the atmosphere, touch organs for sensing movements of masses. The receptive cells of the special sense organs are composed of watery, granular, living substance and elaborate mechanisms have been evolved for converting one or other form of energy into such a form that it can be received by the living substance, e.g. the intricate structure of the eye with its focussing lenses, retinal cells laden with pigment sensitive to light, the ear with its drum membrane vibrating in unison with sound waves in the air, its chain of transmitting osicles, and complicated receiving organ placed in the spiral turns of the cochlea. Be it noted, the receptive cells of the sense organs are immersed in fluid, and each sense organ is specifically sensitive, i.e. only to that form of energy which it has been evolved to receive through countless ages of evolution.